Cavity ringdown spectroscopy: New approaches and outcomes

Cavity ringdown (CRD) absorption spectroscopy provides high sensitivity and photometric precision in the measurement of weak optical absorption spectra. We report several innovations in CRD technique involving either pulsed or continuous-wave (cw) tunable coherent sources. In the former case, we use narrowband, injection-seeded optical parametric oscillator (OPO) devices based on quasi phase-matched nonlinear-optical media and a novel intensity-dip cavity control scheme; one such pulsed OPO is pumped by a compact, inexpensive multimode laser but still delivers single-mode OPO signal output. The latter area concerns cw-CRD spectroscopy with a single-mode tunable diode laser (TDL) and a rapidly swept ringdown cavity, most recently enhanced by a novel optical heterodyne approach that yields a noise-limited absorption sensitivity of 3 × 10^-9 cm^-1. Such pulsed and cw CRD methods are used to record weak rovibrational spectra of carbon dioxide gas (CO₂) with high resolution in the 1.5-μm near-infrared region. Emphasis is on high-performance spectroscopic systems with relatively simple, inexpensive, compact instrumentation.